Power Supply Issues

I have a Dell XPS 1820 and when I connect the supplied PSU to the DC input port, it shows as plugged in but the battery percentage steadily falls as I use the tablet. Booting into diagnostic mode shows that the correct adapter is present. When the battery is exhausted the tablet won't power up.

If I connect the PSU to the supplied desk stand and place the tablet on the stand it connects correctly and starts powering the tablet and charging its battery.

As I cannot always use the stand, this issue is becoming a real problem for me.

Has anyone any ideas as to how to solve this?

Thanks.